X-ray absorption spectroscopy will be used to study biologically-relevant NO binding to heme proteins. Initially, such adducts of heme proteins were the subject of many vibrational, UV/Vis and EPR spectroscopy studies in order to model the binding of dioxygen to heme proteins and find out more about the heme pocket. However, the study of NO adducts has become important in its own right since the discovery that NO binding to hemes has many physiological roles. The present project involves XAS studies of NO binding to nitrophorins, indoleamine-2,3-dioxygenase, leghemoglobin a mutants and horse heart cytochrome c. Edge data will be used to determine the oxidation state of Fe in the 'Fe(III)-NO' adducts that are thought to be best described as Fe(II)-NO+ complexes from Raman spectroscopy, while XAFS will be used to provide precise determinations of Fe-L bond lengths and of the Fe-N-O angle in both the 'Fe(III)' and Fe(II) adducts of the proteins. Variable temperature work will also be performed on model complexes to examine how the angle changes with temperature. These studies will be used to determine more information about the electronic and molecular structures involved in the physiologically-relevant NO binding.